Glucagon-like peptide-1 (GLP-1), also known as proglucagon, induces numerous biological effects such as stimulating insulin secretion, inhibiting glucagon secretion, inhibiting gastric emptying, inhibiting gastric motility or intestinal motility, enhancing glucose utilization, and inducing weight loss. GLP-1 may further act to prevent the deterioration of pancreatic beta-cells that occurs as non-insulin dependent diabetes mellitus progresses. A significant characteristic of GLP-1 is its ability to stimulate insulin secretion without the associated risk of hypoglycemia that is seen when using insulin therapy or some types of oral therapies that act by increasing insulin expression.
The usefulness of therapy involving GLP-1 peptides has been limited by the fact that GLP-1(1-37) is poorly active, and the two naturally occurring truncated peptides, GLP-1(7-37)OH and GLP-1(7-36)NH2, have extremely short in vivo half lives and are rapidly inactivated and cleared. Endogenously produced dipeptidyl-peptidase IV (DPP-IV) inactivates circulating GLP-1 peptides by removing the N-terminal histidine and alanine residues and is a major reason for the short in vivo half-life.
Various approaches have been undertaken to extend the half-life of GLP-1 peptides or reduce clearance of the peptides from the body while maintaining biological activity. U.S. Pat. No. 5,705,483 teaches GLP-1 peptide analogs made resistant to DPP-IV degradation by the incorporation of modifications at the N-terminus of the peptide. An alternative approach for extending the half-life of GLP-1 peptides is derivatization, wherein large acyl groups that prevent DPP-IV from accessing the N-terminus of the peptide are attached to various amino acids of GLP-1 (See International Application No. PCT/DK97/00340, filed Aug. 22, 1997 entitled “GLP-1 Derivatives,” which claims the benefit of DK Provisional Application Nos. 0931/96 filed Aug. 30, 1996, 1259/96 filed Nov. 8, 1996 and 1470/96 filed Dec. 20, 1996). A third approach for extending the half-life of GLP-1 peptides is the covalent attachment of one or more molecules of polyethylene glycol (PEG) to particular residues of a GLP-1 compound to produce a biologically active, PEGylated GLP-1 (See U.S. Pat. No. 7,557,183).
Other therapeutic strategies based on GLP-1 have included the development of enzyme-resistant GLP-1 analogs, such as Exendin-4, a GLP-1 receptor agonist originally isolated from the venom of the Gila monster, with significant homology to native GLP-1. Exendin-4 is resistant to the activity of DPP-IV because of a penultimate glycine residue in place of alanine, and survives longer in circulation. Additionally, small molecule inhibitors, such as DPP-IV inhibitors, have been studied for their ability to extend the metabolic stability and improve the antihyperglycemic and insulinotropic effects of exogenous GLP-1. See, Deacon, Diabetes 53:2181-89 (2004).
In addition to the above, recent attempts have been made to improve the proteolytic resistance and in vivo activity of GLP-1 by preparing variants of GLP-1 such as amino acid substitution and/or synthesis of glycosylated GLP-1 peptides. See Green et al., Biol. Chem., 384:1543-51 (2003); Ueda et al., J. Am. Chem. Soc. 131:6237-45 (2009) prepared glycosylated GLP-1 glycopeptides having one to three glycan groups [GlcNAc, LacNAc and Sialyl LacNAc] added at amino acid residues 19, 26, 34 and 37. However, there remains a critical need for improved GLP-1 peptides with improved metabolic stability and efficacy.